The present invention relates to boats and ships, and more particularly, to boats with auxiliary apparatus for aiding in their steerage.
As a sailboat under way heels to leeward it usually develops weather-helm. Weather-helm is a torque about the vertical axis of the boat that forces the bow to windward unless a counter force is applied by the rudder or other means, Weather-helm is caused by the asymmetrical shape of the heeled hull making way through the water, and by the center of effort on the sails moving outboard of the boat's longitudinal axis as the masts lean with the heeling boat. The magnitude of weather-helm in a given boat generally increases as the boat heels further to leeward.
When sailboats head downwind in heavy weather, they often roll, heeling to windward as well as to leeward. When heeled acutely to windward, the boats often develop substantial lee-helm, which is a torque on the boat in the opposite direction of weather-helm. If this force is not countered quickly, it will induce a dangerous uncontrolled jibe.
Small amounts of weather-helm and lee-helm (hereafter generically referred to as "helm") are seldom a serious problem. But during the severe heeling that is common in heavy weather, many boats develop dysfunctional amounts of helm. Steerage with the boat's rudder is the immediate means used to counter the helm. But as the heel of the boat increases, the rudder becomes less effective in countering the helm because the rudder axis angles obliquely to the water surface and the resulting force on the rudder is divided between a lateral component that corrects the helm and a vertical component that is useless for that purpose. In addition, under extreme angles of heel, part of the rudder often projects out of the water.
Substantial helm slows the boat because of the drag on the acutely turned rudder. It also puts high stress on the rudder and steering means, and strains the helmsperson. Occasionally the helm becomes so strong that it overpowers the rudder and swings the boat out of control.
Several methods are used to reduce extreme helm. One method of reducing weather-helm is to move the sail plan farther forward, or to move the center of lateral resistance of the hull farther aft. If the boat is already well-designed, this will create a lee-helm in light winds which adversely affects the speed and handling of the boat in those conditions. A hull can be configured during the design stage so that it does not develop asymmetrical hydrodynamic forces when heeled. But this usually compromises other design objectives, and it does not eliminate the helm caused by the center of effort on the sails moving outboard as the boat heels. A third approach is to put a trim tab on the aft edge of the keel and rotate it to counter the helm. This approach is expensive, and it requires a complex mechanical linkage to the boat's main steering system to be of use when the boat is heading downwind and rolling substantially. A few boats have been fitted with retractable boards protruding out of the aft edge of the keel or the aft underbody of the hull, increasing the lateral resistance in the aft part of the boat. This solution is also expensive, prone to operating malfunctions, and does not counter the lee-helm caused when rolled to windward. Well-handled boats reduce sail in heavy winds to reduce the heeling, but modest wind velocity changes are frequent at sea and reefing the sails usually requires considerable effort on the part of the crew. A few boats have used dual rudders transversely mounted, or a pendulum rudder that swings to the heeled-down side. Both have real advantages in countering helm, but both are expensive and heavy, and the former adds drag in light winds.
Powerboats, particularly low-speed ones, encounter some steerage difficulties as they heel. The difficulty is seldom as severe as with sailboats. Because the heeling is usually in the form of rolling from one side to the other and because powerboats are not subject to the dangers of an uncontrolled jibe, powerboat helsmpeople usually don't correct with the rudder. Instead, they allow the boat to zigzag back and forth across the intended course. This causes an increase in fuel consumption and in travel time.